In the field of mechanical watches, regulating devices are generally used that are formed of a balance and spring assembly which oscillates and of an escapement, in particular a Swiss lever escapement, comprising a retainer, in particular a lever, and a toothed escapement wheel, the retainer being an intermediate component between the balance and the escapement wheel and which oscillates in synchrony with the balance. The retainer has a to-and-fro movement with rest periods, which produces a jerky advance of the escapement wheel. Said regulating devices, although remarkably improved in the developments of traditional horology, have rather poor efficiency owing in particular to the oscillating movement of the lever with the two extreme rest positions thereof where said lever locks the escapement wheel. Cylinder escapements are also known, which have no lever. In the last case, the device forming a retainer is incorporated in an open tube forming the shaft of the balance. The escapement wheel also has a jerky advance and efficiency is greatly diminished by the fact that the teeth of said escapement wheel press successively and alternately against the inner wall and the outer wall of the open tube.
Devices for regulating the velocity of a wheel, also known as a rotor, by magnetic coupling, also known as magnetic linkage, have also been known for many years, which devices allow the drawbacks of the above-mentioned conventional devices to be overcome. Application in horology is also known, but industrial productions are rare, if not non-existent. Numerous patent applications relating to this field have been filed by Horstmann Clifford Magnetics for the inventions of C. F. Clifford. Documents FR 1.113.932 and U.S. Pat. No. 2,946,183 in particular will be cited. Also known from the Japanese utility model JPS 5263453U (application no. JP19750149018U) is a magnetic escapement of the same type with a direct magnetic coupling between a resonator and an escapement wheel formed by a disk supporting two coaxial annular magnetic tracks. Said two magnetic tracks are substantially contiguous and each comprise wafers with high magnetic permeability forming magnetic zones which are arranged regularly with a given angular period, the wafers of the first track being displaced or shifted by a half-period relative to the wafers of the second track. Non-magnetic zones, in other words zones of low magnetic permeability, are provided between the wafers. Zones of high magnetic permeability are thus obtained distributed alternately on both sides of a circle corresponding to the rest position of at least one magnet carried by the end of a branch of a diapason-type resonator. The magnet of the resonator is coupled magnetically to said two shifted tracks so that said magnet is attracted alternately by the magnetic zones of the first track and of the second track. The escapement wheel therefore rotates with a rotational speed such that said wheel advances by an angular period of the two tracks at each oscillation of the resonator. What is noteworthy in such a system is the absence of mechanical friction in the magnetic escapement and the fact that the escapement wheel has a substantially continuous rotation in a single direction of rotation.
It will be observed that magnetic regulating devices of the above-mentioned type are provided for resonators which have just one degree of freedom for each portion that is subject to a resonance movement. In general, the resonator is arranged so that the magnet, carried by a component that is subject to a resonance movement, oscillates in a substantially radial direction, in other words substantially orthogonal to the two annular magnetic tracks. In this case, the productions of the prior art mentioned have the advantage of having a reduction in frequency between the frequency of the oscillation of the resonator and the rotational frequency (in revolutions/second) of the escapement wheel that carries the magnetic structure. No pivoted mobile rotates or oscillates at a frequency of the order of magnitude of the resonance frequency. The reduction factor is given by the number of angular periods of the annular magnetic tracks.
However, the advantage resulting from a reduction in frequency between the oscillation of the resonator and the rotation of the escapement wheel has a corollary which poses a problem for the magnetic coupling force given the dimensions of a conventional horological movement. The reason is that to increase the frequency reduction, the number of periods of the magnetic tracks must be increased. For a given diameter of the escapement wheel, an increase in the number of periods has the consequence of reducing the surface area of the magnetic zones of the annular tracks. As the magnet of the resonator extends generally over an angular distance of less than a half-period of the annular tracks, the dimensions of said magnet must also be reduced when the frequency reduction increases. It will therefore be understood that the magnetic interaction force between the resonator and the escapement wheel reduces, which limits the torque that can be applied to the escapement wheel and therefore increases the risk of loss of synchronisation between said resonator and said escapement wheel, also known as pulling the regulating system out of synchronisation. Synchronisation here means a given proportional relationship between the resonance frequency and the rotational frequency of the escapement wheel.